In recent years, highly confidential information including My Number (Individual Number) increases and it is expected that it will increase in future. My Number is a public number prescribed for each individual based on the Social Security and Tax Number System (so-called My Number System) in Japan. Various pieces of individual information are associated with My Number. Because of this, My Number is confidential information to avoid leakage. Therefore, it is predicted that technology for concealing the confidential information with higher security strength than before is needed in the future. Normally, when any file containing the confidential information is to be concealed, it is conceivable to encrypt the entire file.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2002-055608
However, when the file containing the confidential information is to be used, there is a case where there is no need to display or describe the confidential information. For example, it is stipulated that a withholding slip does not require My Number to be described when using it for income proof. In this case, there is no need to decode the information of My Number, and the decoding of the information is more likely to cause the risk of information leakage to increase.
For example, as an item, let us consider text files such as a comma-separated values (CSV) file containing confidential information such as My Number. The CSV file is a text file using a comma “,” as a delimiting character (delimiter) for delimiting information for a plurality of items when the information for the items are listed. The CSV file is widely used as a text file expressing a table structure in an input/output format of various applications such as spreadsheet, database, or form, and it is considered that confidential information is included in the file. Conventionally, however, when the CSV file is to be encrypted, the entire CSV file is encrypted. Therefore, only some items of the confidential information are not capable of being flexibly protected.